Reference: OPMAPSTO

args ↦ expr returns the function with the parameter list args (a comma-separated list of identifiers as a string) and expression expr (a string representing a valid expression, possible including the identifiers of args).

A function is represented by a number. Assign this number to a valid identifier, to give the function a symbol.

Examples:  sq ≔ "x" ↦ "x^2"
           geoMean ≔ "a, b" ↦ "sqrt(a⋅b)"